WorldWide Drilling Resource

33 WorldWide Drilling Resource® Don’t Get Stuck in the Cone of Depression Compiled the Editorial Staff of WorldWide Drilling Resource® Well interference occurs when the operation of one or more closely-spaced wells has an adverse effect on another well or perhaps surface water features such as streams or wetlands. This can result in a decline in yield or degradation of well water quality. Most well interference problems tend to be localized and short in duration, but being without water is a major inconvenience and can cause damage to well pumps. When a high-capacity well pumps water, a portion of the aquifer around it is dewatered by causing a reduction in the hydraulic head, known as drawdown. The drawdown generates a funnel-shaped pattern, a cone of depression. Other wells located within the cone of depression may experience lower water levels and have problems getting water if levels drop below the pump in the well. The land area around the cone of depression is called the area of influence. Groundwater flows toward the well into the cone of depression, which can change the natural direction of flow within this area of influence. If the cone of depression for two or more wells overlap, there is said to be well interference, reducing the water available to each of the wells. When well interference occurs, the respective drawdowns are added together, resulting in an accelerated drawdown caused by the interaction of multiple cones of depression. The size of a cone of depression is directly related to the rate and duration of pumping, and the aquifer’s permeability and storage capacity. If large enough, a cone of depression might extend to a nearby stream or lake. The lowered water table below the stream or lake level will begin to lose water to the groundwater aquifer near the well, an occurrence known as induced recharge. In extreme cases, streams and wetlands can be completely dried up by induced recharge from well pumping. Cones of depression are important for a couple of reasons. Because the water flows down to lower points, the cone might change the groundwater flow direction, resulting in undesirable water quality. If there is a source of pollution near the well, the cone might affect whether the pollution flows toward or away from the well. When a large number of wells have been pumping water for a long time, the regional water table can drop significantly. Groundwater mining, as this is called, can necessitate the drilling of deeper, more expensive wells. Mining does not mean groundwater will never be recharged, but in many cases, the recharge rate is negligible on a human timescale. Because of their limited recharge, confined aquifers are more susceptible to groundwater mining. Resolving well interference can sometimes be achieved by lowering the pump in the well or installing a new well pump, but in some situations, it may be necessary to construct a new water supply well. In these cases, the well owner will encounter significant cost. When a suspected case of well interference occurs, a groundwater specialist can investigate the possible connection between the high-capacity pumping and the limited well supply. To determine the extent of interference, it may be necessary to perform an aquifer pumping test and a high-capacity pumping test. Then water levels in the affected well(s) can be measured to determine the severity. Image courtesy of United States Geological Survey. WTR